I'm a computer teacher and want to install Ubuntu in my class (on a temporary hard disk). Now, to install Ubuntu, it takes a while, but I don't want them to just look around in Ubuntu from the disk. I want to let my pupils install Ubuntu, and let them see what it is to install another operating system as Windows.

The only disadvantage is that installing Ubuntu takes a while. After it's been installed, my class is all ready over for quite a while.

Is there any quicker way to let them install Ubuntu, and than subsequently also let them discover the operating system of course?

If you then to just be able to play with the filesystem, Amanda's suggestion of a LiveCD is best.

If you need the full install as part of your lesson I would suggest that you either install Ubuntu(any flavor should work here) with the computer offline or uncheck the box that gives you the option of downloading updates. A full install without the most current updates only takes about 15 minutes, it's the downloading that can be the time killer Esp. if the main servers are busy.

If I remember correctly it it the second screen that has the box you will have a list that states

Computer has 8 GB of disk space

Computer is/is not connected to the internet

at the bottom of this list will be the box that is preselected that states download updates while installing.

Unchecking that box should speed up you install time considerably.

As I stated above, if you just want you students to be able to play around in a Linux filesystem, I would just use a LiveCD as Amanda suggests, the only drawback would be that any changes made would be wiped out on shutdown, if you need the changes to stay you can use a LIveUsb with persistence. The instructions on how to create a LiveUsb with Windows is here, just move the slider in step 4 to create a persistence file.